One of the major airlines was experiencing air-conditioning failure problems with certain of their airplanes and in these particular cases, the failures generally occurred at high altitude and at high airspeed or cruise speed conditions; thereby, causing an altitude restriction to be imposed on the operation of the airplanes. The air-conditioning failure problem was traced to the heat exchanger unit which due to an insufficient cooling air mass flow passing therethrough was overheating and shutting down so that no conditioned air was entering the passenger compartment. The failures became more prevalent on a hot day at high altitude where the airflow into a flush inlet was less dense.
The intake efficiency of a flush inlet deteriorates with increasing thickness of the boundary layer passing over the surface in front of the flush inlet; and this decrease in efficiency is attributable to the ingestion of the slower moving, lower energy boundary layer flow into the flush inlet. A higher energy free stream airflow exists further out from the surface beyond the boundary layer. Therefore, an inlet that is flush with the surface may only receive a fraction of the total pressure i.e., static plus dynamic pressure, existing in the free stream airflow beyond the boundary layer.
An external inlet scoop was designed and tested to improve the flush inlet pressure recovery to acceptable levels and was found to perform very well under the test conditions. However adapting this option to the air-conditioning systems of existing airplanes would prove to be excessively costly and complex. In order to provide sufficient air mass flow into the inlet the height of the scoop was such that it extended into the free stream airflow which produced a drag penalty.
Other fluid flow modifying structures were designed and tested and the most cost effective solution to the air-conditioning failure problems, with the least impact on both the existing airplanes and those in production was to utilize the present invention, comprising a pair of vortex generators located ahead of the flush inlet of the heat exchanger unit.